Carybdea sivickisi is a small (no more than about 14 mm in diameter) sexually dimorphic cubozoan jellyfish with a keyhole rhopalar niche (external opening to the rhopalium) and four distinctive adhesive pads with which the animal attaches to algal substrates during the day (Hartwick 1991; Lewis and Long 2005; Lewis et al. 2008).
Reported from a range of tropical, subtropical, and mild temperate localities in the Pacific, as well as one locality in the Indian Ocean west of Sumatra (see Lewis et al. 2008).
Lewis et al. (2008) note that the timing of maturation of C. sivickisi may vary geographically. In the subtropical waters off Okinawa (Japan), medusae are present between April and early August, when temperatures range between 23 C and 30 C (Lewis and Long 2005). By comparison, in temperate Seto, Wakayama (Japan) juveniles and small mature adults were present at the end of August, when temperatures approached 30 C.
Life History and Behavior
Carybdea sivickisi is largely nocturnal . See Reproduction for a discussion of its mating behavior.
Unlike most jellyfish, which mass spawn (releasing eggs and sperm into the water), Carybdea sivickisi exhibits complex courtship behavior and internal fertilization. Lewis and Long (2005) and Lewis et al. (2008) undertook a detailed investigation of courtship and reproductive behavior of Carybdea sivickisi in the lab. Courtship begins with a female and male swimming close to each other with tentacles fully extended. The male then attaches one of his tentacles to one of the female's tentacles, gaining control of her and pulling her around. The male then contracts his tentacle to bring the oral openings (manubria) of the two individuals into direct contact. Next, the area below each of the males gonads darkens and a strand of red-pigmented sperm is released from each of the eight gonads. These strands coalesce first into four strands in the stomach, and then into a single thick strand in the male's manubrium. Next, the strands are twisted together into a mass called a spermatophore inside the jellyfish's bell. The spermatophore is then transferred to one of the female's tentacles. The male releases the female and she inserts the spermatophore into her manubrium (typically within a few minutes). Two to three days after spermatophore ingestion, thousands of embryos can be seen circulating inside the female, turning the bell an opaque beige color. To expel the embryos, the female begins to pulsate rapidly, and a gelatinous, cream-colored embryo strand is released. Each strand contains over 3,000 embryos, each about 100 microns in diameter (Lewis and Long 2005).
The following account is based on studies in the lab by Lewis and Long (2005): Around 4.5 days after the female has deposited an embryo strand into the substrate, the gelatinous membrane surrounding the embryo strand dissolves and by day 6 the spherical embryos develop into pear-shaped planulae (the typical cnidarian larvae), which are motile (move about). These larvae, each around 140 microns in diameter, settle on the substrate by day 9.5 and by day 14.5 develop into polyps (200 to 700 microns in diameter) with one to ten tentacles. By day 17.5, polyps elongate to become creepers. Lewis and Long (2005) report that in the lab, these creepers often aggregated into long chains (greater than a millimeter in length), which appear to function as a single unit, but the authors suggest that this phenomenon, which has not previously been reported, could be an artifact of rearing conditions. Successful metamorphosis of C. sivickisi polyps into juvenile medusae has not yet been seen in the lab, as the polyps have so far consistently died within a month of settlement on the substrate (Hartwick 1991; Lewis and Long 2005; Lewis et al. 2008).
When juvenile medusae (collected in situ) reach about 4 mm in bell diameter, gonadal development and sexual dimorphism begin to become apparent. Sexually mature females can be recognized (at least under the microscope) by the presence of dark orange markings, known as velar spots, that develop around the margin of the bell (Lewis and Long 2005).
Evolution and Systematics
Systematics and Taxonomy
Based on new data, and with the goal of maintaining the monophyly of Carybdea, Bentlage et al. (2010) suggested that the family Tripedaliidae should be amended to include all former Carybdeidae that have sexually dimorphic gonads, produce spermatophores, and in which at least the males have subgastral sacs/seminal vesicles (Hartwick 1991; Bentlage et al. 2010). Bentlage et al. designated a new genus, Copula, that is defined to contain tripedaliids that have adhesive pads on the exumbrellar apex with which they attach themselves to substrates (Hartwick 1991). The former Carybdea sivickisi would be the type species of this new genus: Copula sivickisi (Stiasny, 1926).